feat: update Bedrock Mantle endpoint and GPT-5.4 model ID (#20109)

## Summary

Amazon Bedrock Mantle's OpenAI-compatible endpoint now lives under
`/openai/v1`, and the GPT-5.4 Mantle model ID no longer uses the `-cmb`
suffix. This updates Codex's built-in Bedrock provider configuration so
generated providers and the static Bedrock catalog use the current
endpoint and model ID.

## Changes

- Update the Bedrock Mantle base URL from
`https://bedrock-mantle.{region}.api.aws/v1` to
`https://bedrock-mantle.{region}.api.aws/openai/v1`.
- Update the Amazon Bedrock default base URL in
`codex-model-provider-info`.
- Change the Bedrock GPT-5.4 catalog slug from `openai.gpt-5.4-cmb` to
`openai.gpt-5.4`.
- Align provider and catalog tests with the new URL and model ID.

## Test Plan

- Manual smoke test:

  ```shell
  target/debug/codex \
      -m openai.gpt-5.4 \
      -c 'model_provider="amazon-bedrock"' \
      -c 'model_providers.amazon-bedrock.aws.region="us-west-2"'
  ```

codex-rs/model-provider/src/provider.rs

@@ -461,7 +461,7 @@ mod tests {
         assert_eq!(
             model_ids,
             vec![
-                "openai.gpt-5.4-cmb",
+                "openai.gpt-5.4",
                 "openai.gpt-oss-120b",
                 "openai.gpt-oss-20b"
             ]
@@ -474,7 +474,7 @@ mod tests {
             .find(|preset| preset.is_default)
             .expect("Bedrock catalog should have a default model");
 
-        assert_eq!(default_model.model, "openai.gpt-5.4-cmb");
+        assert_eq!(default_model.model, "openai.gpt-5.4");
     }
 
     #[tokio::test]